You don't really "need" one but probably the biggest advantage it brings to the table is the ability to use a seperate phase-detect autofocus system. The contrast detection systems that use the normal sensor are slow in comparison, although there are some new hybrid sensors I believe combine phase detection into the main sensor which I assume may work well.

You also get to see exactly what's coming through the lens without the lag or resolution loss of an EVF. I guess the only disadvantage to the viewfinder is in low-light if you can't see the subject, but personally in those situations I'm always using such a long shutter speed I use a tripod and live view anyway.

Out of interest do you own a DSLR or just learning a bit about the differences? If you've only used point & shoots before the speed of a dedicated autofocus system will blow you away especially when combined with a large aperture lens that lets plenty of light in.

I'm wondering how the in-sensor phase-detect system in the Nikon 1 cameras will perform in real life. I wonder if that technology is ready for prime-time.Aside from the phase-detect AF issue, I am also skeptical as to whether electronic viewfinders have proven themselves yet. They also have room for improvement. I don't regard composing via the rear screen as a serious option for a lot of situations, since it is generally difficult to brace a camera in live-view mode, plus having the sensor and rear LCD active chews the battery. (I say it is generally difficult to brace in live-view, since there are a few situations where live-view wins out.)

As a side note, being in New England where winters are long and cold and I like to be out shooting in those conditions...how well would an EVF hold up in sub-zero temperatures? It helps to be able to see what I'm shooting, even if I have to turn the rear LCD off...

* the gopro hero 2 has built-in heat-up function to avoid quick battery run-down at low temperatures; they could add something similar to an EVF if this is really an issue

* the EVF in the new sonys seems to be nearly as good as an OVF (better in some aspects, worse in others, but close anyway); give it one more generation for it to be clearly superior (meaning: bigger on APS-C cameras, no loss of image quality, and lots of extra features, such as zoom-in, peaking, histogram, etc)

* contrast-detect AF is also getting better, give it five years to be just as good as phase-detect AF for everybody except sports shooters

There is EVF in digital camera already. why do we need extra reflex prism inside?

Only a few cameras have a EVF, and so far they have not been up to professional quality, but they are improving.

The resolution is low

They do not keep up with the image as you pan the camera

Reliabiliity should be a little better than those that have been used in many video cameras over several years, its fairly good, but they seem to fail more than I would like.

I believe that they are the wave of the future, but they need to be proven useful and reliable before professionals will want to use them. Optical viewfinders are a mature technology, are extremely reliable and do not require power.

* the gopro hero 2 has built-in heat-up function to avoid quick battery run-down at low temperatures; they could add something similar to an EVF if this is really an issue

Good point. Unlike the rear LCD the EVF is physically small and recessed and I think behind another layer of glass with an air gap (not sure about that) so has some thermal separation from the environment therefore it wouldn't take much power to keep it at least at its minimum operating temp. There are other pros and cons to EVFs though.

* contrast-detect AF is also getting better, give it five years to be just as good as phase-detect AF for everybody except sports shooters

If by "everybody except sports shooters" you mean anyone who doesn't need fast, predictive auto focus, and subject tracking then contrast detection AF is already as good as phase-detect AF and has some advantages. But not sure I would agree that the segment of photographers benefiting from fast AF is so limited... shooting anything that moves from kids running around, people at weddings, photojournalism, sports, wildlife, street photography, etc. is much easier with phase-detection. No matter how good or fast the contrast-detect algorithms can be run, it still has to guess at the direction of focus adjustment, move the focus element a little, measure again, rinse and repeat. That physical process of the adjustment can't be overcome by faster processing.

No matter how good or fast the contrast-detect algorithms can be run, it still has to guess at the direction of focus adjustment, move the focus element a little, measure again, rinse and repeat. That physical process of the adjustment can't be overcome by faster processing.

Could it be helped by faster processing? Take two measurements and extrapolate to the direction and magnitude of adjustment? Maybe that's already being done. I agree that even so, it will never be as fast as phase detect. But maybe fast enough in some cases? AF times and shutter lag on P&S cameras are growing shorter, albeit slowly.

I prefer looking though the old school viewfinder. I rarely go to the live view function. For me, it's just a million times more comfortable and easy to use the viewfinder. I would hate if that was taken away.

No matter how good or fast the contrast-detect algorithms can be run, it still has to guess at the direction of focus adjustment, move the focus element a little, measure again, rinse and repeat. That physical process of the adjustment can't be overcome by faster processing.

Could it be helped by faster processing? Take two measurements and extrapolate to the direction and magnitude of adjustment? Maybe that's already being done. I agree that even so, it will never be as fast as phase detect. But maybe fast enough in some cases? AF times and shutter lag on P&S cameras are growing shorter, albeit slowly.

Or is the answer to build a phase-detect autofocus system into the main imaging sensor?

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Cetalis

If contrast detection AF is too slow, then the EVF refresh rate is probably too slow as well.The resolution is a bit low for double checking AF or manual focusing; it can still be done and theres always magnification or the outlining thingy, but you cant see it all at once. Combined with the higher power draw, this makes it kinda bad for waiting for the right moment, but then again most people don't need to.

No matter how good or fast the contrast-detect algorithms can be run, it still has to guess at the direction of focus adjustment, move the focus element a little, measure again, rinse and repeat. That physical process of the adjustment can't be overcome by faster processing.

Could it be helped by faster processing? Take two measurements and extrapolate to the direction and magnitude of adjustment? Maybe that's already being done. I agree that even so, it will never be as fast as phase detect. But maybe fast enough in some cases? AF times and shutter lag on P&S cameras are growing shorter, albeit slowly.

Or is the answer to build a phase-detect autofocus system into the main imaging sensor?

But then, Nikon claims to use phase-detection AF in their V1 mirrorless although they call it a "hybrid autofocus". I'll look around and try to find an explanation of how they implemented it and if it has limitations compared to the typical phase-detect AF.

They claim AF speed of 0.158 seconds but obviously DSLR AF is much faster since shooting at 10 frames per second and autofocusing in between frames must be less than 0.1 s. So at least in those fuji compacts, that system is not as fast.

as I mostly shoot video, I have an LCD loupe nearly permanently attached to the LCD screen on my T2i, and hardly ever use the OVF (also, all my lenses except one are vintage manual, so I don't use any kind of AF)

once you get used to having all the goodies that an EVF has (in terms of information displayed alongside your image, and accessibility of the menus and options), it is hard to go back to an OVF, specially if it is a small one like the one on my T2i, or any other APS-C camera for that matter (with the louple, my "EVF" is significantly bigger than the OVF on a 5D2)

that's why I think EVF will be prevalent in the near future (given how well the new sony ones perform: resolutions seems to be no problem anymore, the only bit that still needs improvement seems to be the refresh rate)

the only bit that could be a bigger problem is AF, so I think in less than 5 years all manufacturers will go SLT for the high-end models and also some mid-end ones (some people need really fast AF, and/or will settle for nothing but the best) and mirrorless for everything else (as soon as contrast-detect AF becomes good enough for most)